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Site Research Part 1: A Natural History of the Flint Hills

February 24, 2011
Map of the Flint Hills

(from "Geology and the Prairie" by the National Parks Service)

Because of their unique terrain, the Flint Hills of Kansas are home some of the last remaining tallgrass prairie in North America, which once covered 140 million acres extending from Canada to Texas. The hills are composed of sedimentary rock deposited by the a shallow inland waterway between 250 and 300 million years ago. The map below reveals that Permian bedrock (blue) is only exposed in a narrow north-south strip in eastern Kansas. West of the Flint Hills, the Permian bedrock is overlain by sediments deposited between 125 and 2.5 million years ago by the uplift and then erosion of the Rocky Mountains.

Surficial Geology of Kansas

Generalized Surficial Geology of Kansas (based on a drawing by the Kansas Geological Survey)

The following series of images are taken from “Paleogeography and Geologic Evolution of North America,” a series of 100 digitally rendered images of North American geology over the past 550 million years, created by Ron Blakey at the Northern Arizona University.

Middle Permian (275 million years ago)

Middle Permian (275 million years ago)

The expansion and regression of this inland waterway over millions of years resulted in a cyclothem of alternating beds of limestone and shale in what is today eastern Kansas. Limestone is largely composed of skeletal remains of marine microorganisms, while shale is the result of depositions of mud and silt on the sea floor. Several of the limestone strata are embedded with nodules of extremely hard crystalline quartz, known as chert or flint, for which the hills are named.

Late Cretaceous (100 million years ago)

Late Cretaceous (100 million years ago)

Cretaceous bedrock was laid in western Kansas in a similar process by the Western Interior Seaway, which opened up as the Farallon oceanic plate slid beneath the North American Plate in the process that led to the uplift of the Rocky Mountains.

Neogene Miocene (15 million years ago)

Neogene Miocene (15 million years ago)

The uplift of the Rocky Mountains occurred over more than 30 million years starting about 80 million years ago. As wind and water eroded this enormous upland, debris flowed across Kansas in the form of vast sloping sheet of sand and gravel that ended against the Permian uplands that would become the Flint Hills. The Ogallala formation, (composed of semi-consolidated clay, silt, sand, gravel, and caliche) underlies western Kansas, Nebraska, and Northern Texas, and contains one of the world’s largest aquifers.

Present Day

Present Day

During the past several million years, wind and waterborne sand, silt, and volcanic ash have continued to spill eastward into Kansas, resulting in a fertile topsoil.

The Evolution of the Flint Hills

Formation of the Flint Hills

Formation of the Flint Hills (drawing by Boris Tchatalbachev)

Water eroded Permian bedrock of the Flint Hills over millions of years, producing a gently sloped topography. The differential erosion of the durable limestone and the softer shale has resulted in benched hillsides, illustrated below::

Benched topography in the Flint Hills

Benched topography in the Flint Hills (from "Geology and the Prairie" by the National Parks Service)

Benched Topography

Limestone benches in the Flint Hills. The Lower Fox Creek School (1882) is visible in the lower left, was built of Cottonwood Limestone taken from the Barney Lantry quarry (photo by James Klauder)

Much of the limestone, particularly the Florence Limestone which has already eroded from everywhere but the highest hilltops, contained a great deal of chert nodules (flint), which remain scattered across the surface. The presence of the flint made the hillsides impervious to steel plows, rendering the landscape unsuitable to farming and thus preserving the tallgrass prairie ecology.

Chert

Chert (photo by James Klauder)

The Evolution of the Tallgrass Prairie

Historic Extent of the Prairie

Historic Extent of the Prairie (from 100 Common Wildflowers of the Tallgrass Prairie, by Susan Lamb)

Around 18,000 years ago, most of North America east of the Rocky Mountains was covered in spruce and jack pine in the north and oak and hickory around the Gulf of Mexico. A global warming trend starting around 10,000 years ago began to make much of the land within the rain shadow of the Rocky Mountains too dry to support trees. As the forests retreated eastward, grasslands developed in their place. This continued for about 5,000 years, at which point a cooling trend resulted in a more humid climate capable of supporting trees west of the Mississippi. By this time, however, Native American’s had discovered the advantages of annually burning the prairie grasses, which promoted rapid growth during the spring and increased the population of wild buffalo. This burning prevented trees from taking root anywhere but watery lowlands and thus artificially prevented the return of widespread forestation. When Europeans began settling west of the Mississippi in the 1800s, most of the tallgrass prairie was plowed under to grow crops.

Paleovegetation Maps for North America

In the Flint Hills, the scattered chert made plowing impossible, which led to a culture of grazing. Cattle ranchers recognized that burning the grasses benefited their herds, continuing that tradition that has made the Flint Hills on of the last remaining stretches of tallgrass prairie.

(Research for this post by Boris Tchatalbachev, Jared Marcantoni, and James Klauder)

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